- A
Use the show interfaces serial0/0/0 command to verify the interface status and check for encapsulation mismatch.
The show interfaces serial 0/0/0 command displays the interface status, including whether it is up/up, administratively down, or has encapsulation errors. This helps identify if the serial link is operational and if the encapsulation (e.g., PPP or HDLC) matches the neighbor.
- B
Use the show ip route command to verify that the route to 192.168.2.0/24 is present in the routing table.
Why wrong: While show ip route is useful for checking routing table entries, the question specifically suspects an interface issue on the serial link, not a missing route. The route may still be present if OSPF learned it via another path, but the serial link issue would prevent communication.
- C
Use the ping 10.0.0.2 command to test Layer 3 connectivity to the neighbor router.
Why wrong: Pinging the neighbor's IP address tests Layer 3 connectivity, but if the serial interface is administratively down or has an encapsulation mismatch, the ping will fail. However, the question asks for a command to verify the interface issue, not just test connectivity.
- D
Use the show running-config interface serial0/0/0 command to check the configuration of the serial interface.
Why wrong: While show running-config interface serial 0/0/0 displays the configuration, it does not show the operational status or real-time errors. The show interfaces command is more appropriate for verifying the interface state and encapsulation.
Troubleshooting Serial Interface Problems: Clock Rate and Encapsulation
This 200-301 practice question tests your understanding of network infrastructure and connectivity. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
You are connected to the console of R1. The network administrator reports that users cannot communicate with the server at 192.168.2.10. R1 is connected to R2 via a serial link (S0/0/0) with IP 10.0.0.1/30 on R1 and 10.0.0.2/30 on R2. The network uses OSPF for routing. You suspect an interface issue on the serial link.
Quick Answer
The answer is to use the show interfaces serial0/0/0 command to verify the interface status and check for encapsulation mismatch. This is correct because serial interface troubleshooting often reveals that the link is down due to an incorrect clock rate on the DCE side or a mismatch in Layer 2 encapsulation, such as one side running PPP while the other uses HDLC. On the CCNA 200-301 v2 exam, this scenario tests your ability to isolate serial link failures in an OSPF environment, where a down interface prevents neighbor adjacency and breaks connectivity to the server at 192.168.2.10. A common trap is assuming the IP addressing is wrong when the real issue is a missing clock rate or encapsulation mismatch, so always start with show interfaces serial to check line protocol status. Remember the memory tip: “Clock on DCE, encaps match—or your serial link will catch.”
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Use the show interfaces serial0/0/0 command to verify the interface status and check for encapsulation mismatch.
The serial interface may be administratively down or have incorrect encapsulation. Setting the clock rate on the DCE side and ensuring PPP encapsulation matches the neighbor resolves the issue.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Use the show interfaces serial0/0/0 command to verify the interface status and check for encapsulation mismatch.
Why this is correct
The show interfaces serial 0/0/0 command displays the interface status, including whether it is up/up, administratively down, or has encapsulation errors. This helps identify if the serial link is operational and if the encapsulation (e.g., PPP or HDLC) matches the neighbor.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
Use the show ip route command to verify that the route to 192.168.2.0/24 is present in the routing table.
Why it's wrong here
While show ip route is useful for checking routing table entries, the question specifically suspects an interface issue on the serial link, not a missing route. The route may still be present if OSPF learned it via another path, but the serial link issue would prevent communication.
- ✗
Use the ping 10.0.0.2 command to test Layer 3 connectivity to the neighbor router.
Why it's wrong here
Pinging the neighbor's IP address tests Layer 3 connectivity, but if the serial interface is administratively down or has an encapsulation mismatch, the ping will fail. However, the question asks for a command to verify the interface issue, not just test connectivity.
- ✗
Use the show running-config interface serial0/0/0 command to check the configuration of the serial interface.
Why it's wrong here
While show running-config interface serial 0/0/0 displays the configuration, it does not show the operational status or real-time errors. The show interfaces command is more appropriate for verifying the interface state and encapsulation.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Use the show interfaces serial0/0/0 command to verify the interface status and check for encapsulation mismatch.Correct answer▾
Why this is correct
The show interfaces serial 0/0/0 command displays the interface status, including whether it is up/up, administratively down, or has encapsulation errors. This helps identify if the serial link is operational and if the encapsulation (e.g., PPP or HDLC) matches the neighbor.
✗Use the show ip route command to verify that the route to 192.168.2.0/24 is present in the routing table.Wrong answer — click to see why▾
Why this is wrong here
The show ip route command does not provide interface-level details such as encapsulation or clock rate; it only shows routing information.
Why candidates choose this
Candidates often default to checking the routing table when there is a connectivity issue, but the question explicitly points to a serial interface problem.
✗Use the ping 10.0.0.2 command to test Layer 3 connectivity to the neighbor router.Wrong answer — click to see why▾
Why this is wrong here
Ping does not provide detailed interface status or configuration information; it only indicates whether the neighbor is reachable, not why it is not.
Why candidates choose this
Ping is a common first step in troubleshooting, but the question specifically asks to verify an interface issue, which requires interface-level commands.
✗Use the show running-config interface serial0/0/0 command to check the configuration of the serial interface.Wrong answer — click to see why▾
Why this is wrong here
The running-config shows the intended configuration but not the current operational state; for example, it won't show if the interface is administratively down unless you check the shutdown command.
Why candidates choose this
Candidates may think checking the configuration is sufficient, but the question asks to 'verify' the issue, which implies checking the operational status.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Command / output trap
While show ip route is useful for checking routing table entries, the question specifically suspects an interface issue on the serial link, not a missing route. The route may still be present if OSPF learned it via another path, but the serial link issue would prevent communication.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 200-301 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Use the show interfaces serial0/0/0 command to verify the interface status and check for encapsulation mismatch. — The serial interface may be administratively down or have incorrect encapsulation. Setting the clock rate on the DCE side and ensuring PPP encapsulation matches the neighbor resolves the issue.
What should I do if I get this 200-301 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 200-301 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 7, 2026
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